Add function for size-checked conversion of Integral types

Based on the discussion in this thread, I would like to add a function to the base library that is similar to fromIntegral but only successful if the argument fits in the result type.

If possible, I would like to get this into 7.10. My apologies for running late on it. Hopefully, since it is a relatively small change overall, the "only" controversy will be bikeshedding.

I have concluded that adding @hvr's intCastMaybe from int-cast is the best possible option. Previously, I thought a Bounded-based version was also useful; however, I realized that it did not deal optimally with conversions like Int<->Word/Int8<->Word8/etc. as well as intCastMaybe does and would need specialized versions that intCastMaybe provides automatically.